From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D0979390991 for ; Tue, 3 Mar 2026 00:20:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772497221; cv=none; b=tqSGS1xfjYZAsyFWbQgz8TncUnp6dELOSyCNnOsQNTw9iTTx9OascBlsntHFFa2/NyvszmUkbIigMVccip8CUzxCR33miTE6lCi6CxwXGh1mri3yOnXFjKcqYFLRzYv3IKd1oEexIAFW6neGgQH6R3/biPdUeIDoQY/XVCAIvlQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772497221; c=relaxed/simple; bh=qusRBsBs2gBLCAXQDdHP7X/vGLqql3bRPAfzrJIV79g=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=SSaJzBS0FEHOCxcfpP4fiMpXcX8lvUfsYYKHhNQp1ycWDeIFxmMfiV9XQmS9bCAkN8UfqRXoJFXtwNx+teY19q7zujyIzY3ksUv+mAcvQ3Zm+srb51M5qZlhWjmIXIcce0syR5G4wHXHE20cmWhPsKI6EpxSYcpV+xZCOJ3Fazo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=YjYi1HH1; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="YjYi1HH1" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1772497218; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=+PxAs2WvNVOxCftZ/jr64yaJOBUiDO5P1MZH42ytmr4=; b=YjYi1HH1JVPTq/97ZLb2aX0aIH4Y9Yj4ZpFAMGHmCSbqTxIzsaJGxBtn+lI33pd7bvjYtA xrKXHJ+o6OIZ025oUPR2VY9J6OrS08bDe4Ta3T7w0Fk0sbP1d55XjIQB8Ra6p3AsLjaIK9 6TboqnNvsxJ4QCcPWquWLG4ZUoSvqzc= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-377-6OCu4L77MnWbFpTG33YaJw-1; Mon, 02 Mar 2026 19:20:15 -0500 X-MC-Unique: 6OCu4L77MnWbFpTG33YaJw-1 X-Mimecast-MFC-AGG-ID: 6OCu4L77MnWbFpTG33YaJw_1772497213 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 1BBD21800366; Tue, 3 Mar 2026 00:20:12 +0000 (UTC) Received: from tpad.localdomain (unknown [10.96.133.6]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id BC4521800348; Tue, 3 Mar 2026 00:20:09 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 17973401CFE28; Mon, 2 Mar 2026 21:19:44 -0300 (-03) Date: Mon, 2 Mar 2026 21:19:44 -0300 From: Marcelo Tosatti To: Leonardo Bras Cc: Michal Hocko , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, Johannes Weiner , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Vlastimil Babka , Hyeonggon Yoo <42.hyeyoo@gmail.com>, Leonardo Bras , Thomas Gleixner , Waiman Long , Boqun Feng , Frederic Weisbecker Subject: Re: [PATCH 0/4] Introduce QPW for per-cpu operations Message-ID: References: <20260206143430.021026873@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 On Fri, Feb 27, 2026 at 10:23:27PM -0300, Leonardo Bras wrote: > On Mon, Feb 23, 2026 at 10:06:32AM +0100, Michal Hocko wrote: > > On Fri 20-02-26 18:58:14, Leonardo Bras wrote: > > > On Mon, Feb 16, 2026 at 12:00:55PM +0100, Michal Hocko wrote: > > > > On Sat 14-02-26 19:02:19, Leonardo Bras wrote: > > > > > On Wed, Feb 11, 2026 at 05:38:47PM +0100, Michal Hocko wrote: > > > > > > On Wed 11-02-26 09:01:12, Marcelo Tosatti wrote: > > > > > > > On Tue, Feb 10, 2026 at 03:01:10PM +0100, Michal Hocko wrote: > > > > > > [...] > > > > > > > > What about !PREEMPT_RT? We have people running isolated workloads and > > > > > > > > these sorts of pcp disruptions are really unwelcome as well. They do not > > > > > > > > have requirements as strong as RT workloads but the underlying > > > > > > > > fundamental problem is the same. Frederic (now CCed) is working on > > > > > > > > moving those pcp book keeping activities to be executed to the return to > > > > > > > > the userspace which should be taking care of both RT and non-RT > > > > > > > > configurations AFAICS. > > > > > > > > > > > > > > Michal, > > > > > > > > > > > > > > For !PREEMPT_RT, _if_ you select CONFIG_QPW=y, then there is a kernel > > > > > > > boot option qpw=y/n, which controls whether the behaviour will be > > > > > > > similar (the spinlock is taken on local_lock, similar to PREEMPT_RT). > > > > > > > > > > > > My bad. I've misread the config space of this. > > > > > > > > > > > > > If CONFIG_QPW=n, or kernel boot option qpw=n, then only local_lock > > > > > > > (and remote work via work_queue) is used. > > > > > > > > > > > > > > What "pcp book keeping activities" you refer to ? I don't see how > > > > > > > moving certain activities that happen under SLUB or LRU spinlocks > > > > > > > to happen before return to userspace changes things related > > > > > > > to avoidance of CPU interruption ? > > > > > > > > > > > > Essentially delayed operations like pcp state flushing happens on return > > > > > > to the userspace on isolated CPUs. No locking changes are required as > > > > > > the work is still per-cpu. > > > > > > > > > > > > In other words the approach Frederic is working on is to not change the > > > > > > locking of pcp delayed work but instead move that work into well defined > > > > > > place - i.e. return to the userspace. > > > > > > > > > > > > Btw. have you measure the impact of preempt_disbale -> spinlock on hot > > > > > > paths like SLUB sheeves? > > > > > > > > > > Hi Michal, > > > > > > > > > > I have done some study on this (which I presented on Plumbers 2023): > > > > > https://lpc.events/event/17/contributions/1484/ > > > > > > > > > > Since they are per-cpu spinlocks, and the remote operations are not that > > > > > frequent, as per design of the current approach, we are not supposed to see > > > > > contention (I was not able to detect contention even after stress testing > > > > > for weeks), nor relevant cacheline bouncing. > > > > > > > > > > That being said, for RT local_locks already get per-cpu spinlocks, so there > > > > > is only difference for !RT, which as you mention, does preemtp_disable(): > > > > > > > > > > The performance impact noticed was mostly about jumping around in > > > > > executable code, as inlining spinlocks (test #2 on presentation) took care > > > > > of most of the added extra cycles, adding about 4-14 extra cycles per > > > > > lock/unlock cycle. (tested on memcg with kmalloc test) > > > > > > > > > > Yeah, as expected there is some extra cycles, as we are doing extra atomic > > > > > operations (even if in a local cacheline) in !RT case, but this could be > > > > > enabled only if the user thinks this is an ok cost for reducing > > > > > interruptions. > > > > > > > > > > What do you think? > > > > > > > > The fact that the behavior is opt-in for !RT is certainly a plus. I also > > > > do not expect the overhead to be really be really big. > > > > > > Awesome! Thanks for reviewing! > > > > > > > To me, a much > > > > more important question is which of the two approaches is easier to > > > > maintain long term. The pcp work needs to be done one way or the other. > > > > Whether we want to tweak locking or do it at a very well defined time is > > > > the bigger question. > > > > > > That crossed my mind as well, and I went with the idea of changing locking > > > because I was working on workloads in which deferring work to a kernel > > > re-entry would cause deadline misses as well. Or more critically, the > > > drains could take forever, as some of those tasks would avoid returning to > > > kernel as much as possible. > > > > Could you be more specific please? > > Hi Michal, > Sorry for the delay > > I think Marcelo covered some of the main topics earlier in this > thread: > > https://lore.kernel.org/all/aZ3ejedS7nE5mnva@tpad/ > > But in syntax: > - There are workloads that are projected not avoid as much as possible > return to kernelspace, as they are either cpu intensive, or latency > sensitive (RT workloads) such as low-latency automation. > > There are scenarios such as industrial automation in which > the applications are supposed to reply a request in less than 50us since it > was generated (IIRC), so sched-out, dealing with interruptions, or syscalls > are a no-go. In those cases, using cpu isolation is a must, and since it > can stay really long running in userspace, it may take a very long time to > do any syscall to actually perform the scheduled flush. > > - Other workloads may need to use syscalls, or rely in interrupts, such as > HPC, but it's also not interesting to take long on them, as the time spent > there is time not used for processing the required data. > > Let's say that for the sake of cpu isolation, a lot of different > requests made to given isolated cpu are batched to be run on syscall > entry/exit. It means the next syscall may take much longer than > usual. > - This may break other RT workloads such as sensor/sound/image sampling, > which could be generally ok with some of the faster syscalls for their > application, and now may perceive an error because one of those syscalls > took too long. > > While the qpw approach may cost a few extra cycles, it operates remotelly > and makes the system a bit more predictable. > > Also, when I was planning the mechanism, I remember it was meant to add > zero overhead in case of CONFIG_QPW=n, very little overhead in case of > CONFIG_QPW=y + qpw=0 (a couple of static branches, possibly with the > cost removed by the cpu branch predictor), and only add a few cycles in > case of qpw=1 + !RT. Which means we may be missing just a few adjustments > to get there. Leo, v2 of the patchset adds only 2 cycles to CONFIG_QPW=y + qpw=0. The larger overhead was due to migrate_disable, which is now (on v2) hidden inside the static branch. My bad. > BTW, if the numbers are not that great for your workloads, we could take a > look at adding an extra QPW mode in which local_locks are taken in > the fastpath and it allows the flush wq to be posponed to that point in > syscall return that you mentioned. What I mean is that we don't need to be > limitted to choosing between solutions, but instead allow the user (or > distro) to choose the desired behavior. > > Thanks! > Leo I think 2 cycles is acceptable.